Pressure sensing devices have been known and used extensively in the petrochemical, process, and other industries involving fluid systems. In general, a pressure sensor is responsive to a fluctuating sensed fluid pressure. The level of pressure is transduced within the sensor, which either enables or blocks the flow of a pilot fluid through a passageway in the sensor. Thus, such devices act as a variable position valve. Pressure sensors are useful, for instance, as automatic shuf-off devices to prevent the continued flow of a inflammable or explosive fluid during a fire or other emergency. Alternatively, a pressure sensor may be used to activate an automatic sprinkler system in a similar situation.
The operation of conventional pressure sensors is usually accomplished by using a piston and cylinder assembly. The piston is exposed to the fluctuating sensed fluid pressure which urges the piston in one direction along the cylinder. A spring, or like resilient device, urges the piston in the opposite direction. Equilibrium of these forces at any given moment determines the position of the piston within the cylinder. By calibrating these positions, it is possible to accurately transduce the pilot pressure into a particular position of the piston.
Typically, the piston may include grooves on its outside surface. The cylinder includes offset input, output, and vent ports for the passage of the pilot fluid. Passage of fluid from one port to another is normally blocked by the presence of the piston. The piston grooves may be sealed from each other and so constructed in relation to the distances offsetting the ports that communication is enabled between only two of the ports at a time through the space within the cylinder defined by the grooves. When the sensed fluid is pressurized with a desired range, the piston is positioned so as to allow the flow of the sensed fluid from the input port, through the device and exiting through the output port. When exposed to pilot pressures other than those desired, the piston will assume a position which will block the passage of the pilot fluid and enables communication between the outlet port containing residual pilot fluid to a vent port.
Frequently, it is necessary to modify the response characteristics of a pressure sensor to accomodate varying processes and operations. U.S. Pat. No. 4,091,832, issued to Snyder et al, discloses a high or low only pressure sensing valve in which the characteristics of the device may be altered by increasing or decreasing the force exerted by the spring. The force exerted on the spring is adjusted by rotating a threaded end cap which constrains the spring within the cylinder, thereby increasing or decreasing its compression. The response of the valve may also be varied by disconnecting the pressure sensor and replacing the lower portion thereof containing the piston with a corresponding module containing a piston of smaller or larger diameter. The smaller the diameter, and therefore the area, the smaller the force exerted by the sensed fluid on the piston.
U.S. Pat. No. 4,258,741, issued to Roger, again shows a spring loaded piston-cylinder device wherein the tension exerted by the spring may be similarly externally adjusted by rotation of a threaded end cap which varies the compression on the spring. The opposing force exerted on the piston by the pilot fluid may be altered by modifying the effective diameter of the piston. This is, accomplished by constructing a piston of two components. The first component is a cylinder slidingly engaged within the cylinder having a concentric opening defined therewithin. The opening is comprised of two regions of staggered diameters. A second component with conforming characteristics is slidably engaged within the opening. By dismantling the pressure sensor device and altering the orientation of the piston, the effective surface area may be suitably altered.
Conventional pressure sensors combining high and low sensing capabilities generally utilize two separate springs for adjustment of response characteristics. One spring is adjustable to modify low pressure response characteristics, while the second spring is similarly adjustable and acts with the first spring to control high pressure response.
However, these and other conventional designs exhibit several inefficiencies. Although the ability to alter the compression on a spring or springs is a useful means of affecting the response characteristics of a pressure sensor, frequently it is inadaquate to accommodate the extreme variation in sensed fluid pressure levels and does not enable control over the range of pressures within which the sensor allows the passage of pilot fluid therethrough. The fact that the grooves formed in the surface of the piston in existing devices are of fixed length precludes any possibility of affecting this response range.
Further, the process of disconnecting the pressure sensor from the fluid system, removing and replacing a given piston with one of a different diameter, or one formed with grooves of different lengths, or altering the effective diameter of the piston, is inherently inefficient and labor intensive. Physically interchanging or modifying components of the sensor necessarily entails extended downtime for the fluid system and the construction and maintenance of an extensive and costly inventory of piston modules of varying characteristics. Since each piston is contructed of a fixed diameter and contour, a finite set of such pistons are able only to accomodate a discrete number of response ranges and levels. It is frequently desirable to adjust the pressure sensor over a continuous and infinitely variable range of response levels, and it is also desirable to be able to adjust the response without dismantling or removing the pressure sensor from the input and out flow fluid conduits and without disturbing the operation of the overall system.
Therefore a principal feature and advantage of the present invention is to provide a high and low pressure sensor wherein the range of desired operating pressures is continuously and infinitely variably adjustable.
It is another feature and advantage of the invention to provide a pressure sensor which enables its response characteristics to be altered without disconnecting the sensor from the fluid system.
It is yet another feature and advantage of the invention to provide a pressure sensor which may be adjusted without disrupting the operation of the fluid system.
It is another feature and advantage of the invention to provide a more economical high and low pressure sensor with a reduced number of components.
Therefore, these and other features and advantages of the invention will become more clearly evident upon a detailed examination of the following drawings, claims, and description, wherein like numerals denote like parts in the several views.